After computing the energy deposition, the ionization and the thermal balance
are solved in NLTE [1]}. Ionization is assumed to be entirely due to impact with the high-energy particles produced by the deposition of the radioactive products, while photoionization is assumed to be negligible
[2]}. The rate of impact ionization and the recombination
rate are balanced for each ion to compute the degree of ionization. Level
populations are computed solving the rate equations under the assumption of
thermal balance, i.e.,  equating the non-thermal heating rate and the rate of
cooling via line emission. Under the assumption that the nebula is optically
thin, radiation transport is not performed. The resulting line emissivity is
used to compute the emerging spectrum.
